The refining of petroleum crude oils to obtain a variety of lubricating oils which function effectively in various environments has become a highly developed process. Although the broad principles involved in refining are qualitatively understood, there are quantitative uncertainties which require considerable resort to empiricism in practical refining. Underlying these quantitative uncertainties is the complexity of the molecular constitution of lubricating oils. Because lubricating oils for the most part are based on petroleum fractions boiling above about 232.degree. C. (450.degree. F.), the molecular weight of the hydrocarbon constituents is high and these constituents display many conceivable structures and structure types. This complexity and its consequences are referred to in "Petroleum Refinery Engineering,: by W. L. Nelson, McGraw Hill Book Company, Inc., New York, NY 1958 (Fourth Edition).
In the production of lubricants a suitable crude oil which contains a quantity of lubricant base stock having a predetermined set of properties such as, for example, viscosity, oxidation stability, and maintenance of fluidity at low temperatures is subjected to a set of subtractive unit operations which removes the unwanted components. The most important of these unit operations include distillation, solvent refining, and dewaxing, which except for catalytic dewaxing, are physical separation processes in the sense that if all the separated fractions were recombined the original crude would be reconstituted.
A lubricant base stock (i.e., a refined oil) may be used as such as a lubricant, or it may be blended with another lubricant base stock having somewhat different properties. Or, the base stock, prior to use as a lubricant, may be compounded with one or more additives which function, for example, as antioxidants, extreme pressure additives, and viscosity index (V.I.) improvers. As used herein, the term "stock", regardless whether or not the term is further qualified, will refer only to a hydrocarbon oil without additives. The term "raw stock" will be used herein to refer to an untreated viscous distillate or the residuum fraction of crude petroleum oil isolated by vacuum distillation of a reduced crude from atmospheric dilstillation, or its equivalent. The term "solvent-refined stock" or "raffinate" will refer to an oil that has been solvent extracted, for example, with furfural. The term "dewaxed stock" will refer to an oil which has been treated by any method to remove or otherwise convert was contained therein and thereby reduce its pour point. The term "waxy," as used herein, will refer to an oil of sufficient was content to result in a pour point greater than -4.degree. C. (25.degree. F.). The term "stock", when unqualified, will be used to refer to the viscous fraction in any stage of refining, but in all cases free of additives. The term "base stock" will refer to an oil refined to a point suitable for some particular end use, such as for preparing automotive oils.
Conventional practice for the preparation of high grade mineral lubricating oil base stocks is to vacuum distill an atmospheric tower residuum from an appropriate crude oil as the first step. This step provides one or more raw stocks typically within the boiling range of about 290.degree. to 565.degree. C. (550.degree. to 1050.degree. F.) and a vacuum residuum. Each raw stock is then extracted with a solvent, e.g., furfural, phenol or N-methyl pyrrolidone, which is selective for aromatic hydrocarbons, and which removes the undesirable aromatic components. The vacuum residuum usually requires an additional deasphalting step to remove asphaltic material prior to solvent extraction. The raffinate from solvent refining is then catalytically dewaxed.
In recent years techniques have become available for catalytic dewaxing of petroleum stocks. A process of that nature developed by British Petroleum is described in The Oil and Gas Journal dated Jan. 6, 1985, at pages 69-73. See also U.S. Pat. No. 3,668,113. In U.S. Pat. No. Re. 28,398 is described as a process for catalytic dewaxing with a catalyst comprising zeolite ZSM-5. Such a process combined with catalytic hydrofinishing is described in U.S. Pat. No. 3,894,938. Catalytic dewaxing of raffinate using intermediate pore size zeolite catalysts such as ZSM-5 is described in U.S. Pat. Nos. 4,181,598 and 4,259,170.
The manufacture of lube oil base stocks is designed to produce a lube product according to very strict specifications for pour point, viscosity, viscosity index (V.I.) and product flash point. Often, improvements in one or more of these product parameters can be achieved only by adversely affecting other product parameters. However, if improvements can be realized in the product viscosity index, for instance, without adversely affecting the remaining product parameters, substantial benefits may accrue. Similarly, if an improvement in the downstream processing of the lube oil can be achieved so as to produce a lube oil stock with one or more properties exceeding the required specifications, the refinery operator is provided with a greater variety of options on how to exploit the improved performance, with the potential for improving the economic performance of the refinery.